This project is directed at delineating the cellular and molecular mechanisms of the immunopathogenesis of human immunodeficiency virus (HIV) infection. We have continued to investigate the inductive and suppressive effects of immunoregulatory cytokines on HIV expression. Using a model of transforming growth factor beta (TGF-Beta) suppression of phorbol myristate acetate (PMA) induction of HIV expression in chronically infected promonocytic cells (U1) we have demonstrated both tumor necrosis factor (TNF)-dependent and TNF-independent pathways of HIV induction as well as NF-KappaB dependent and independent pathways of virus induction and suppression. We demonstrated that the anti-oxidants glutathione (GSH) and N-acetyl cysteine (NAC) could block the induction of HIV expression by PMA or cytokines. Unlike GTH, NAC actually blocked transcription of HIV mRNA. Retinoic acid suppressed induction of HIV in a manner strikingly similar to that of TGF-Beta suggesting its possible use as a therapeutic agent. We demonstrated that glucocorticoids synergized with TNF-Alpha and interleukin-6 (IL-6) to induce HIV expression from U1 cells by different post-transcriptional mechanisms. Using a modified polymerase chain reaction assay to detect HIV-specific mRNAs we demonstrated that active viral replication occurs at all stages of HIV disease including the early asymptomatic stage. In followup of our previous observations that thymocyte precursors were susceptible to HIV infection, we observed that thymic epithelial (TE) cells are not susceptible to HIV infection. However, normal human serum induces TE cells to secrete IL-6 which upregulates the expression of HIV in infected cells suggesting that these cells might actually promote the intrathymic spread of HIV. We demonstrated that CD34+ bone marrow progenitor cells from a subset of seropositive individuals most of whom have advanced immunosuppression are infected with HIV in vivo. We demonstrated that lymphoid organs are major reservoirs of HIV and lymph node CD4+ cells have at least a 10x greater frequency of infection than peripheral blood CD4+ cells. Finally, LFA-1 regulates HIV-mediated fusion and syncytia formation in human CD4+ cells.